16 Studies on Vegan Diets — Do They Really Work?
Vegan diets are becoming increasingly popular.
They claim to offer various health benefits, ranging from weight loss and reduced blood sugar to prevention of heart disease, cancer and premature death.
However, most studies on vegan diets are observational. This makes it difficult to know if the benefits observed are actually caused by the vegan diet itself.
This article analyzes 16 randomized controlled studies — the gold standard in scientific research — to evaluate how a vegan diet can affect your health.
1. Wang, F. et al. Effects of Vegetarian Diets on Blood Lipids: A Systematic Review and Meta-Analysis of Randomized Controlled Trials. Journal of the American Heart Association, 2015.
Details: This meta-analysis of 11 randomized controlled studies included 832 participants.
The studies lasted from three weeks to 18 months and evaluated changes in participants' total, LDL, HDL, non-HDL cholesterol and triglyceride levels.
Seven of the included vegetarian studies looked specifically at vegan diets, and each of them had a control group.
Results: Vegetarian diets lowered total, LDL, HDL and non-HDL cholesterol more than the control diets, but they did not affect blood triglyceride levels.
Conclusions: Vegetarian diets effectively lowered blood levels of total, LDL, HDL and non-HDL cholesterol more than the control diets.
2. Macknin, M. et al. Plant-Based, No-Added-Fat or American Heart Association Diets: Impact on Cardiovascular Risk in Obese Children with Hypercholesterolemia and Their Parents. The Journal of Pediatrics, 2015.
Details: Thirty obese children with high cholesterol levels and their parents were recruited for the study. Each pair was randomly assigned to follow either a vegan diet or an American Heart Association (AHA) diet for 4 weeks.
Both groups attended weekly classes and cooking lessons specific to their diet.
Results: Total calorie intake significantly decreased in both diet groups.
Children and parents following the vegan diet consumed less protein, cholesterol, saturated fat, vitamin D and vitamin B12, and they consumed more carbs and fiber than those in the AHA group.
Children following the vegan diet lost 6.7 lbs (3.1 kg) over the four-week study period, which was 197% more than those in the AHA group.
Children in the vegan group reduced their systolic blood pressure, total and LDL cholesterol levels, whereas those in the AHA groups didn't. However, the improvements weren't large enough to reach statistical significance.
At the end of the study, children following the vegan diet had significantly lower BMIs than those following the AHA diet.
Parents in the vegan groups had 0.16% lower hemoglobin A1C levels, which are used as a measure of blood sugar control, as well as lower total and LDL cholesterol levels than those on the AHA diet.
Those parents also lost 3.5 lbs (1.6 kg) more than parents on the AHA diet. However, the difference wasn't large enough to reach statistical significance.
Conclusions: Both diets lowered heart disease risk in children and adults. However, the vegan diet more greatly affected the children's weight and the parents' cholesterol and blood sugar levels.
3. Mishra, S. et al. A multicenter randomized controlled trial of a plant-based nutrition program to reduce body weight and cardiovascular risk in the corporate setting: the GEICO study. European Journal of Clinical Nutrition, 2013.
Details: 291 participants were recruited from 10 GEICO corporate offices. Each office was paired with another, and employees from each paired site were randomized to either a low-fat vegan diet or a control diet for 18 weeks.
Participants in the vegan group were provided weekly support group classes led by a dietitian. They took a daily vitamin B12 supplement and were encouraged to favor low-glycemic index foods.
Participants in the control group made no dietary changes and didn't partake in weekly support group sessions.
Results: The vegan group consumed more fiber and less total fat, saturated fat and cholesterol than the control group.
Participants who completed the 18-week study lost 9.5 lbs (4.3 kg) if they were in the vegan group, compared to 0.2 lbs (0.1 kg) if they were in the control group.
Total and LDL cholesterol levels dropped by 8 mg/dL in the vegan group, compared to almost no change in the control groups.
HDL cholesterol and triglyceride levels both increased more in the vegan groups than in the control.
Blood pressure fell slightly in both groups. Hemoglobin A1C levels dropped by 0.7% in the vegan group, compared to 0.1% in the control group.
Conclusions: Participants in the vegan groups lost more weight. They also improved their blood cholesterol and blood sugar levels compared to those following a control diet.
4. Barnard. N. D. et al. The effects of a low-fat, plant-based dietary intervention on body weight, metabolism, and insulin sensitivity. The American Journal of Medicine, 2005.
Details: 64 overweight, post-menopausal women were recruited. Each woman was randomly assigned to follow either a low-fat vegan or a low-fat control diet based on the National Cholesterol Education Program (NCEP) guidelines for 14 weeks.
No calorie restrictions were used, and both groups were encouraged to eat until they were full. Participants prepared their own meals and attended weekly nutritional support groups for the duration of the study.
Results: Despite no overt calorie restriction, both groups consumed around 350 fewer calories per day. The vegan group consumed less dietary protein, fat and cholesterol and more fiber than the NCEP diet group.
Participants in the vegan group lost an average of 12.8 lbs (5.8 kg), compared to 8.4 lbs (3.8 kg) in those following the NCEP diet. Changes in BMI and waist circumference were also greater in the vegan groups.
Blood sugar levels, fasting insulin and insulin sensitivity improved significantly for all.
Conclusions: Both diets improved markers of blood sugar control. However, the low-fat vegan diet helped overweight, post-menopausal women lose more weight than the low-fat NCEP diet.
5. Turner-McGrievy, G. M. et al. A Two-Year Randomized Weight Loss Trial Comparing a Vegan Diet to a More Moderate Low-Fat Diet. Obesity, 2007.
Details: This study was based on the same 64 overweight, post-menopausal women in the study above who were randomly assigned to a low-fat vegan or a low-fat NCEP diet for 14 weeks.
This study was done in two cohorts. All participants were offered weekly group nutrition support for the first 14 weeks of the study.
However, the first cohort didn't receive any nutritional support after the first 14 weeks, whereas the rest continued with bimonthly support group meetings for one year.
All women were followed for two years. No participant was prescribed any calorie restriction goals, and both groups were encouraged to eat until they were full.
Results: The vegan group lost 10.8 lbs (4.9 kg) after one year, compared to 4 lbs (1.8 kg) in the NCEP group.
Over the next year, both group regained some weight. At the end of the two-year study, the weight loss was 6.8 lbs (3.1 kg) in the vegan group and 1.8 lbs (0.8 kg) in the NCEP group.
Regardless of the diet assignment, the women who received group support sessions lost more weight than those who didn't receive them.
Conclusions: Women on a low-fat vegan diet lost more weight after one and two years, compared to those following a low-fat diet. Also, women receiving group support were better able to lose weight and maintain it.
6. Barnard, N.D. et al. A Low-Fat Vegan Diet Improves Glycemic Control and Cardiovascular Risk Factors in a Randomized Clinical Trial in Individuals With Type 2 Diabetes. Diabetes Care, 2006.
Details: 99 participants with type 2 diabetes were recruited and pair-matched based on their hemoglobin A1C levels.
Each pair was then randomly assigned to follow a low-fat vegan diet or a diet based on the 2003 American Diabetes Association (ADA) guidelines for 22 weeks.
Portion sizes, calorie intake and carbs were unrestricted on the vegan diet. Those on the ADA diet were instructed to cut 500–1,000 calories per day from their normal diet.
Everyone received a vitamin B12 supplement, and alcohol was limited to one serving per day for women and two servings per day for men.
All participants were also provided an initial one-on-one session with a registered dietitian and attended weekly nutrition group meetings for the duration of the study.
Results: Both groups reduced calorie intakes by approximately 400 calories per day, despite only the ADA group being instructed to do so.
Protein and fat intakes also decreased in both groups. However, participants in the vegan group consumed 152% more carbs than the ADA group.
Participants following the vegan diet doubled their fiber intake, whereas the amount of fiber consumed by those in the ADA group remained the same.
By the end of the 22-week study period, the vegan group lost 12.8 lbs (5.8 kg), which was 134% more weight than the ADA group.
Both groups reduced their total cholesterol, LDL and HDL cholesterol levels, but no differences were observed between groups.
What's more, the vegan participants' hemoglobin A1C (HbA1c) levels dropped by 0.96 points, which was 71% more than the ADA participants' levels.
The graph below shows the HbA1c changes in the vegan diet groups (blue) and ADA diet groups (red).
Conclusions: Both diets helped participants lose weight and improve their blood sugar and cholesterol levels. However, the vegan diet caused more weight loss and a greater reduction in blood sugar than the ADA diet.
7. Barnard, N.D. et al. A low-fat vegan diet and a conventional diabetes diet in the treatment of type 2 diabetes: a randomized, controlled, 74-wk clinical trial. American Journal of Clinical Nutrition, 2009.
Details: This study included the type 2 diabetics from the previous study, randomized to follow either a low-fat vegan diet or an ADA diet.
After the initial 22-week intervention period, all participants were given the option to continue with group sessions for an additional 52 weeks.
Results: By the end of the 74-week study period, 17 participants in the vegan group reduced their diabetes medication dosages, compared to 10 in the ADA group.
Participants in the vegan group also lost 3 lbs (1.4 kg) more weight than those following the ADA diet, but the difference wasn't statistically significant.
Hemoglobin A1C levels, which are used as a marker for blood sugar control, decreased more in participants in the vegan group.
In addition, LDL- and total cholesterol levels decreased by 10.1 – 13.6 mg/dL more in the vegan groups than in the ADA group.
Conclusions: Both diets improved type 2 diabetics' blood sugar and cholesterol, but the vegan diet affected these levels more. Both diets helped participants lose weight, but differences between diets weren't significant.
8. Nicholson, A. S. et al. Toward Improved Management of NIDDM: A Randomized, Controlled, Pilot Intervention Using a Low-Fat, Vegetarian Diet. Preventive Medicine, 1999.
Details: 11 participants with type 2 diabetes were recruited and randomly assigned to a low-fat vegan diet or a conventional low-fat diet.
All participants were offered prepared lunches and dinners according to their diet specifications for a total of 12 weeks.
Participants were also allowed to prepare their own meals if they preferred, but researchers reported that most used the catered meal option.
Because of its lower fat content, participants on the vegan diet consumed around 150 fewer calories per meal than those on the conventional diet.
All participants attended an initial half-day orientation session, as well as support group sessions every other week for the duration of the study.
Results: Participants in the vegan group reduced their fasting blood sugar levels by 28%, compared to a 12% decrease in those following the conventional low-fat diet.
Those on the vegan diet also lost an average of 15.8 lbs (7.2 kg) over the 12-week study period, compared to an average of 8.4 lbs (3.8 kg) for the conventional dieters.
No differences in total and LDL cholesterol were noted, but HDL cholesterol levels fell in the vegan group.
Conclusions: A low-fat vegan diet decreased fasting blood sugar levels and helped participants lose more weight than a conventional low-fat diet.
9. Turner-McGrievy, G. M. et al. Low glycemic index vegan or low-calorie weight loss diets for women with polycystic ovary syndrome: a randomized controlled feasibility study. Nutrition Research, 2014.
Details: 18 overweight and obese women with polycystic ovarian syndrome (PCOS) were recruited. Each was randomly assigned to follow a low-fat vegan diet or a low-calorie diet for six months.
Results: Women in the vegan group lost a total of 1.8% of their body weight over the first three months, compared to 0% in the low-calorie group. However, no significant differences were observed after six months.
Interestingly, participants with higher engagement in a Facebook support group lost more weight than the rest.
Those following the vegan diet consumed an average of 265 fewer calories than those on the low-calorie diet, despite not being given a specific lower-calorie goal.
Participants in the vegan group also consumed less protein, less fat and more carbs than those following the low-calorie diet.
No differences were observed in pregnancy or PCOS-related symptoms between the two groups.
Conclusions: A vegan diet is more effective at naturally reducing the amount of calories eaten per day, despite the lack of a calorie restriction goal. It may also help women with PCOS lose weight.
10. Turner-McGrievy, G. M. et al. Comparative effectiveness of plant-based diets for weight loss: a randomized controlled trial of five different diets. Nutrition, 2015.
Details: 50 overweight adults were recruited and randomized to follow one of five low-fat, low-glycemic index diets for six months. The assigned diets were either vegan, vegetarian, pesco-vegetarian, semi-vegetarian or omnivorous.
Participants were given instructions related to their respective diets by a registered dietitian and encouraged to limit processed and fast food.
All participants except those in the omnivorous diet group attended weekly group meetings. The omnivore group attended monthly sessions and received the same diet information through weekly emails instead.
All participants consumed a daily vitamin B12 supplement and had access to private Facebook support groups.
Results: Participants in the vegan group lost an average of 7.5% of their body weight, which was the most of all groups. In comparison, omnivores lost only 3.1%.
The vegan diet group consumed more carbs, as well as fewer calories and fat than omnivores, despite not having been given any specific calorie or fat restriction goals.
Protein intakes were not significantly different between groups.
Conclusions: Vegan diets can result in greater weight loss than vegetarian, pesco-vegetarian, semi-vegetarian or omnivorous diets.
Details: 106 type 2 diabetics were randomly assigned to a 12-week vegan or conventional diet recommended by the Korean Diabetes Association (KDA).
Calorie intake was not restricted for either group over the 12-week study period.
Results: Participants in the vegan group naturally consumed an average of 60 fewer calories per day, compared to the conventional diet group.
Hemoglobin A1C levels decreased in both groups. However, those in the vegan group reduced their levels by 0.3–0.6% more than the conventional diet group.
Interestingly, BMI and waist circumference decreased only in the vegan group.
There were no significant changes in blood pressure or blood cholesterol levels between groups.
Conclusions: Both diets helped type 2 diabetics' blood sugar control. However, the vegan diet improved it more than the conventional diet. Also, a vegan diet was more effective at reducing BMI and waist circumference.
12. Belinova, L. et al. Differential Acute Postprandial Effects of Processed Meat and Isocaloric Vegan Meals on the Gastrointestinal Hormone Response in Subjects Suffering from Type 2 Diabetes and Healthy Controls: A Randomized Crossover Study. PLoS ONE, 2014.
Details: 50 type 2 diabetics and 50 healthy subjects were randomly assigned to consume either a protein and saturated fat-rich pork burger or a carb-rich vegan couscous burger.
Blood concentrations of sugar, insulin, triglycerides, free fatty acids, gastric appetite hormones and oxidative stress markers were measured before the meals, as well as up to 180 minutes after the meals.
Results: The two different meals produced similar blood sugar responses in both groups over the 180-minute study period.
High insulin levels persisted for longer after the meat meal than the vegan meal in both healthy participants and type 2 diabetics.
Triglyceride levels increased and free fatty acids decreased to a higher extent following the meat meal. This happened in both groups, but the difference was more obvious in type 2 diabetics.
The meat meal produced a greater decrease in the hunger hormone ghrelin than the vegan meal, but only in healthy participants. In diabetics, ghrelin levels were similar after both types of meals.
Additionally, the meat meal led to greater increases in markers of cell-damaging oxidative stress than the vegan meal, but only in diabetics.
Antioxidant activity increased following the vegan meal, but only in healthy controls.
Conclusions: In healthy individuals, vegan meals reduce hunger less but increase antioxidant activity more. Meat meals cause more oxidative stress in diabetics and a greater requirement for insulin.
13. Neacsu, M. et al. Appetite control and biomarkers of satiety with vegetarian (soy) and meat-based high-protein diets for weight loss in obese men: a randomized crossover trial. The American Journal of Clinical Nutrition, 2014.
Details: 20 obese men were randomly assigned to consume either a vegetarian or meat-based, high-protein weight loss diet for 14 days.
After the first 14 days, the diets were switched so that the vegetarian group received the meat-based diet for the following 14 days and vice versa.
Diets were calorie-matched and provided 30% of calories from protein, 30% from fat and 40% from carbs. The vegetarian diet was based on soy protein.
All food was supplied by the dietetic research staff.
Results: Both groups lost around 4.4 lbs (2 kg) and 1% of their body weight, regardless of the diet consumed.
No differences in hunger ratings or the desire to eat were noted between the groups.
The pleasantness of the diets was rated high for all meals, but participants generally rated the meat-containing meals higher than the soy-based vegan ones.
Both diets reduced total, LDL and HDL cholesterol, as well as triglycerides and glucose. However, the decrease in total cholesterol was significantly greater for the soy-based vegan diet.
Levels of ghrelin were slightly lower in the meat-based diet, but the difference wasn't large enough to be considered significant.
Conclusions: Both diets had similar effects on weight loss, appetite and gut hormone levels.
14. Clinton, C. M. et al. Whole-Foods, Plant-Based Diet Alleviates the Symptoms of Osteoarthritis. Arthritis, 2015.
Details: 40 participants with osteoarthritis were randomly assigned to consume either a whole-food, plant-based vegan diet or their regular omnivorous diet for six weeks.
Participants in both groups were encouraged to eat freely and not count calories. Both groups prepared their own meals for the duration of the study.
Results: Participants in the vegan group reported greater improvements in energy levels, vitality and physical functioning, compared to the regular diet group.
The vegan diet also resulted in higher scores on self-rated functioning assessments among participants with osteoarthritis.
Conclusions: A whole-food, plant-based vegan diet improved symptoms in participants with osteoarthritis.
15. Peltonen, R. et al. Faecal microbial flora and disease activity in rheumatoid arthritis during a vegan diet. British Journal of Rheumatology, 1997.
Details: 43 participants with rheumatoid arthritis were randomly assigned to consume either a raw, vegan diet rich in lactobacilli or their habitual omnivorous diet for one month.
The participants in the vegan group received pre-packed, probiotic-rich raw meals for the duration of the study.
Gut flora was measured through stool samples. Disease activity was evaluated through the use of several questionnaires.
Results: The participants consuming the probiotic-rich, raw vegan diet had a significant change in their fecal flora during the study period.
No changes were observed in the fecal flora of the participants on the habitual omnivorous diet.
Participants in the probiotic-rich, raw vegan group experienced significantly more improvements in disease symptoms, such as swollen and tender joints.
Conclusions: A probiotic-rich, raw vegan diet changes gut flora and decreases symptoms of rheumatoid arthritis more effectively than a standard omnivorous diet.
16. Nenonen, M.T. et al. Uncooked, lactobacilli-rich, vegan food and rheumatoid arthritis. British Journal of Rheumatology, 1998.
Details: This study used the same 43 participants with rheumatoid arthritis as in the study above.
Participants were randomly assigned to follow a lactobacilli-rich, raw vegan diet or continue with their existing omnivorous diet for 2–3 months.
Those in the vegan group received pre-packed, probiotic-rich raw meals for the duration of the study.
Results: The participants in the raw vegan group lost 9% of their body weight, while the control group gained 1% of their body weight, on average.
By the end of the study, blood protein and vitamin B12 levels slightly decreased, but only in the raw vegan group.
Participants in the raw vegan group reported significantly less pain, joint swelling and morning stiffness than those continuing with their existing diet. A return to their omnivorous diet aggravated their symptoms.
However, when more objective indicators of rheumatoid arthritis symptoms were used, no difference in symptoms was observed between groups.
Some of the participants following the probiotic-rich, raw vegan diet reported symptoms of nausea and diarrhea, which caused them to withdraw from the study.
Conclusions: A probiotic-rich, raw vegan diet increased weight loss and improved subjective disease symptoms in those with rheumatoid arthritis.
10 of the randomized controlled trials above examined the effects of a vegan diet on weight loss.
Seven out of the 10 studies reported that a vegan diet was more effective than the control diet at helping participants lose weight.
In the most impressive study, the vegan diet was able to help participants lose 9.3 more pounds (4.2 kg) than the control diet over an 18-week period (3).
That said, when diets were matched for calories, the vegan diet was no more effective than the control diet for weight loss (13).
Unfortunately, not many studies described whether the weight loss came from the loss of body fat or the loss of body muscle.
Despite being generally higher in carbs, vegan diets were up to 2.4 times more effective at improving blood sugar control in diabetics, compared to control diets.
In fact, seven out of eight randomized controlled studies reported vegan diets to be more effective than conventional ones, including diets recommended by the ADA, AHA and NCEP.
The one study that didn't find the vegan diet to be better reported it to be as effective as the control diet (12).
The greater weight loss generally reported in participants following the vegan diet could further contribute to the blood sugar-lowering effects.
In total, 14 studies examined the effects of vegan diets on blood cholesterol levels.
However, the effects on HDL cholesterol and triglyceride levels are mixed. Some studies reported increases, others decreases and some no effect at all.
Only two of the studies looked at the effects of vegan diets on appetite and satiety.
The first reported that a vegan meal reduced the hunger hormone ghrelin less than a meat-containing meal in healthy participants, whereas the second reported no difference between a vegan meal and a meat-containing meal in diabetics (12, 13).
So far, only three randomized controlled studies looked at the effects of a vegan diet on osteo- or rheumatoid arthritis.
Vegan diets seem to be very effective at helping people lose weight.
They also seem to be well suited to reduce symptoms of arthritis.
In addition, there's scientific evidence to support that vegan diets greatly reduce blood sugar, total cholesterol and LDL cholesterol.
Based on the best available evidence, a vegan diet can be very healthy if done right.